with the battery expanding and shrinking during use they couldn’t keep it from rattling around unless they glued it down.

I’ve never designed mobile hardware, but it seems like the easy fix for that would be to glue the battery to a thin backplane and then screw the backplane down; then people could just replace the battery+backplane as a single unit…

(ETA: but I’ll take a Steam Deck with a non-replaceable battery over any of the existing competition any day.)